With the development and advancement of technology, liquid crystal display (LCD) has replaced the traditional cathode ray tube (CRT) display extensively in the computer, communication and consumer electronics industry. Compared to the traditional CRT display, LCD display has the advantages of being thinner and lighter with low radiation, thus liquid crystal display panels may be suitable for popular electronic products, such as notebooks, personal digital assistants (PDA), mobile phones, digital cameras, flat panel televisions, projectors, digital camcorders, and digital photo frames etc.
Inside the LCD display, back lights of the display are provided by the back light module. Generally speaking, in order to allow users to view the display of liquid crystal display panel clearly under all types of environment, the luminance of the back light needs to be adjustable. Under outdoor lighting environments, the luminance of the back light needs to be brighter than the background lighting, and under darker environments, the luminance of the back light needs to be lowered for providing a softer lighting to reduce eye fatigue. The luminance of liquid crystal displays are usually controlled by the regulation of on/off timing ratio for the back light or by the regulation of electric current going through the back light. The back light module of liquid crystal display panels usually comprises a light source and an inverter circuit for driving the light source. The inverter circuit converts the input direct-current voltage (DC voltage) to alternating-current voltage (AC voltage), and utilizes the AC voltage to drive the light source. As known to those skilled in the art, circuit designers may utilize the principles of LC oscillation to convert direct current to alternating current.
In addition, a popular method for controlling the luminance of a light source is to regulate the pulse width (time period) of the AC voltage supplied to a lamp based on the signals provided by a pulse width modulator (PWM). The wider the pulse width of the AC voltage, the brighter the luminance of the lamp, on the contrary, the narrower the pulse width of the AC voltage, the darker the luminance of the lamp. However, the dimming circuit for controlling luminance of a light source that utilizes the aforementioned LC oscillation principles and pulse width modulators suffers the drawback of having difficulty with lowering the luminance when the lamp is turned off (i.e. PWM signal equals 0) due to LC oscillation. Refer to FIG. 1, it illustrates the output voltage versus time (where time is measured in milliseconds) graphs for the pulse width modulator and the inverter of the dimming circuit for controlling luminance of lamp according to prior art. As shown in FIG. 1, when the signal of the pulse width modulator equals 1, the inverter outputs the AC voltage to the lamp, and controls the luminance of the lamp via time period W (pulse width) of the PWM signal. However, as the PWM signal switches to 0, the output voltage of the inverter needs to endure a period of voltage oscillation before the voltage may be returned to 0 volts, making it difficult to lower the luminance of the lamp.
Refer to FIG. 2, it illustrates the output voltage versus time (where time is measured in microseconds) graphs for the pulse width modulator and the inverter of the dimming circuit for controlling luminance of lamp according to prior art. As shown in FIG. 2, when the PWM signal switches from 1 to 0, voltage oscillation of the inverter may be observed.
U.S. Pat. No. 5,939,830 has disclosed a method and apparatus for dimming a lamp in a backlight of a liquid crystal display, which reduces the period of voltage oscillation. Refer to FIG. 3, it illustrates the circuit diagram for controlling luminance of a lamp according to prior art (U.S. Pat. No. 5,939,830). When switch S1 of dimming circuit 300 is closed (on-state), DC power +V will be applied to the center tap 346 of transformer 340 via inductor L1. As capacitor C1 is connected in parallel to the primary winding 342 of transformer 340, in coordination with the on and off of switches S2 and S3 (controlled by the switch controller 330) will generate LC oscillation between primary winding 342 and capacitor C1. Thus, DC power +V is converted to AC power, and the AC power is applied to the device to be controlled 310 via the secondary winding 344 of transformer 340. In order to control the luminance of the device to be controlled 310, dimming circuit 300 utilizes the pulse width modulator 320 to control the on and off periods of switch S1 for regulating the “on-time” of the device to be controlled 310.
In order to improve on the LC oscillation problem that occurs when the PWM signal is switched from 1 to 0, a switch S4 controlled by the output signal 324 of pulse width modulator 320 is added between inductor L1 and center tap 346. As switch S1 is opened (output signal 322 equals 0), output signal 324 switches from 0 to 1, which closes switch S4 (on-state). Thus, the energy stored at primary winding 342 will be directed to the ground, which greatly reduces the voltage oscillation period for the device to be controlled 310.
However, as the abovementioned solution relies on the connection of primary winding 342 to the ground to improve the effects of LC oscillation, which would lead to a waste of energy, thus the lower the luminance of the device to be controlled 310, the lower the electrical efficiency of the back light module. Also, even if switch S4 is closed at the right moment, LC oscillation would still occur as capacitor C1 is still connected electrically to the primary winding 342 of transformer 340. Therefore, the voltage at the device to be controlled 310 would still need to endure a certain period for voltage to return to 0 volts. As the result, when the luminance of the device to be controlled 310 is low, it becomes more difficult to further lower the luminance of the device to be controlled 310.
The previously described prior art also mentioned that switch S4 may be connected across the secondary winding 344 of transformer 340, thus that the energy stored within transformer 340 is dissipated to ground when switch S1 is opened. This still leads to a waste of energy, and consequently the lower the luminance of the device to be controlled 310, the lower the electrical efficiency of the back light module. Therefore, switch S4 needs to be able to sustain high voltages, thereby increasing the cost of the dimming circuit for controlling luminance of lamps.
Due to the aforementioned problems, the present invention provides a dimming circuit for controlling luminance of light source and the method for controlling luminance. The present invention has the effects of raising the electrical efficiency of the back light module during low luminance, achieves the goal of making it easier to lower the luminance of lamps with a lower cost, and reduces the waste of energy.